In discrete multitone (DMT) systems there is usually employed a number of discrete carrier channels e.g. 256 spaced apart by a fixed amount e.g. 4.3125 KHz. According to ITU conventions all 256 channels carry data except one, typically the 64th which is a pure tone used to synchronize the receiver clock with the transmitter using phased lock loop circuits to insure accuracy of frequency and phase and of frame definition between the transmitter and receiver. These systems work well for small phase error but when a larger, micro-interruption occurs the errors can exceed 360° so that the circuit, e.g. a modem cannot detect whether the error was just ε or ε+360° or ε+720° . . . . When this occurs the entire modem or other system must be turned off and then on again to allow the modem to completely retrain itself. The use of another channel providing a second pure tone would allow for a detection of errors over 360°, 720° or greater but conventional protocols do not permit that.
Separately, even though the receiver corrects the synchronization clock a few thousand times per second the temperature drift, and other effects can be too much to adjust for and the error can exceed the bounds of 360°, 720° . . . . An attempt to correct the higher drift introduces jitter into the clock signal, which is undesirable.